I have a tank with three goldfish in it. Some weeks ago, I had to do a 100% water change and had to take the fishes out. While returning them to the tank, one of them, an Oranda, jumped off and hit the floor. There was no apparent injury and the damage to his Wen seemed minimal. However, within two or three hours after falling, the part of his Wen on the side that hit the floor started bleeding internally and soon completely covered that portion in blood. Four or five hours later, however, the bleeding subsided and the Wen looked almost completely normal, save for the very minor deformity. Afterward I noticed that, for the next few hours, the Oranda would swim in circles at the lowest level of the tank and in the same direction.
After the incident, the Oranda's behavior seemed to have changed and now he spends most of the day sitting at the bottom of the tank, which is a departure from his usual behavior as he had always been the most active swimmer in the tank. I was concerned that he may have developed swim bladder disease as a result of falling onto the floor, but he would swim around the tank occasionally, especially during feeding time.
I also noticed that even though he would still eat during feeding time, he seemed to have trouble locating the flakes in the water even when the filtration would push them into the water. Sometimes he would approach one of the flakes floating in the water only to stopped and lose interest once it gets close enough to his face.
Some weeks later, I noticed that there was something different about one of his eyes, and observed that there was a small bulge behind it that was gradually enlarging. So I got a new tank to use as a quarantine tank and put Aquarium Salt in it.
After two days, the swelling seemed to have gone down a bit, and I put him back in the main tank where after a day, he seemed to have gone back to being active, spending around half of the day or more swimming around in the tank and only sometimes sitting inactive at the bottom. This lasted for about two days.
Now, he seems to be back to spending most of the day at the bottom of the tank. In addition, I've observed that the swelling behind his eye has worsened and has begun pushing the eye outward.
Besides all these symptoms, he appears to be pretty healthy and shows no apparent sign of distress and discomfort. Could his fall have caused an injury that I don't know about and cause the change in his behavior? Would an infection in his Wen from the damage be immediately visible or could I unaware of a possible infection here? What can I do the fix his worsening popeye before it gets worse?
UPDATE: I had added some plants to the main tank and would use this every other day. The Oranda seems to be much better now and is swimming around and foraging in the tank with the other goldfish like he used to. The swelling behind his eye is still there and seems to be very gradually receding.